This invention relates generally to the deposition of conductive films on a substrate. More particularly, it relates to chemical vapor deposition methods for forming relatively thick refractory metal films on a semiconductor surface.
In the development of VLSI technology, there is a continuous trend to reduce the dimensions of the microelectronic devices present on the semiconductor chip and in this way increase the speed of operation. In the microelectronics industry, refractory metals, such as tungsten and molybdenum, have been studied for use as conducting films, however, current deposition technologies suffer from several disadvantages.
Refractory metals for VLSI applications are generally applied according to one of three different methods: sputtering, evaporation, and chemical vapor deposition. The primary advantage of a sputtering or evaporation process is the ability to apply most metals, rather than only the group which have volatile reaction products. However, sputtering and evaporation often require complicated and expensive equipment and are prone to nonuniform coverage when topology is severe over the wafer.
Chemical vapor deposition (CVD) of refractory metals offers several advantages over sputtering and evaporation. CVD of refractory metals can provide better coverage, reduced system complexity, and higher purity deposits. Also in some applications, selective CVD does not require an additional photolithography step. The selective chemical vapor deposition process is a process in which the refractory metal, or other material, is deposited only on areas with certain chemical reactivities. For example, tungsten hexafluoride will react with silicon or polysilicon gates, but not with the surrounding silicon dioxide isolation areas.
However, tungsten films formed in the past by CVD methods have suffered from a number of limitations. Tungsten films formed by the hydrogen reduction of tungsten hexafluoride, according to the equation, EQU 3H2+WF6.fwdarw.W+6HF.uparw. (1)
produce hydrofluoric acid as a by-product. This is undesirable since the HF tends to etch away the silicon dioxide area surrounding the polysilicon gate, potentially destroying the device. The thickness of films produced by this method is difficult to reproduce under the best of conditions, and the film surface is frequently rough. The tungsten films formed by the hydrogen reduction method are highly stressed which can cause delamination of the films from the substrate. Deposition rates are relatively slow using this method; 30 to 50 angstroms of tungsten is deposited per minute at temperatures below 400 degrees centigrade. A slow rate is relatively disadvantageous when a thick tungsten film is desired. Also, after 1000 A have been deposited, the process is no longer selective, and tungsten will deposit on the silicon dioxide or other surrounding areas.
Tungsten films have also been formed by the silicon reduction of tungsten hexafluoride according to the equation. EQU 2WF6+3Si.fwdarw.2W+3SiF4.uparw. (2)
The by-product from this reaction is silicon tetrafluoride which is volatile but generally nonreactive with semiconductor materials. Also, the rate of deposition is much greater than that for the hydrogen reduction process, on the order of 500 to 600 angstroms of tungsten deposited per minute. However, this reaction has two major disadvantages. First, like the hydrogen reduction method, the films produced by this method are highly stressed. When the silicon atom is replaced tungsten by WF6, the larger tungsten atoms introduce stress in the film. Second, the silicon reduction method requires that silicon be available in order for the reaction to take place. As the tungsten is deposited, less and less silicon is available from the underlying area. This causes the reaction to be self-limiting, typically only films of 300 to 400 angstroms can be deposited. One prior art method discloses that an argon plasma treatment may be used to increase nucleation sites on the silicon for tungsten deposition, and thereby increasing the available silicon. However, this technique will only extend the silicon reduction method to form films of approximately 1000 angstroms. Beyond this thickness, other means of depositing tungsten are required.